Firstly, the international administration and co-ordination of the ISBN system is done through the International Standard Book Number Agency, based in Berlin, Germany. In turn, the International ISBN agency is assisted by the International Organization for Standardization (ISO) together with the International Publishers Association (IPA), the International Federation of Library Associations (IFLA), and individual ISBN group agencies. For in-depth information you should visit www.isbn.org.

An International Standard Book Number (ISBN) identifies an edition of a monographic work. The ISBN is defined by the standard NISO/ANSI/ISO 2108:1992 [ISO1]. While ISBN references are typically found on books, they are also assigned to audio-books and the like as well - the qualifying characteristic of a product being that it is "book like."

Basically, an ISBN is a ten-digit number (actually, the last digit can be the letter "X" as well, as described below), or more recently also a 13-digit number, which is divided into four variable length parts usually separated by hyphens when printed.
The parts are as follows (in this order):

a group identifier which specifies a group of publishers, based on national, geographic or some other criteria,

the publisher identifier,

the title identifier,

and a modulus 11 check digit, using X instead of 10.

The group and publisher number assignments are managed in such a way that the hyphens are not needed to parse the ISBN unambiguously into its constituent parts. However, the ISBN is normally transmitted and displayed with hyphens to make it easy for human beings to recognize these parts without having to make reference to or have knowledge of the number assignments for group and publisher identifiers.

There are however, rules concerning the hyphenation of ISBN references, which dictate where the hyphens should be placed, separating the reference into four distinct parts. Read more about the rules of ISBN hyphenation.

There are plans, which are now in effect as of January 1st, 2007, to extend the ISBN into 13 digits in order to make the system more suitable for identification of electronic monographs. The so-called Bookland ISBN consists of a traditional ISBN preceded by the 978 or 979 EAN flag. More information about the creation and conversion of the ISBN to ISBN Bookland EAN.

Books which were originally issued with an ISBN-10 code will keep this code; in turn these same ISBN-10 references can also be converted into ISBN-13 codes. However, new books issued with the new-style ISBN-13 references cannot be converted back to ISBN-10 references. ISBN-10 codes which have been converted into ISBN-13 codes, can easily be identified because their new, 13-digit counterpart will begin with "978". Likewise, the codes for new books which have not previously been assigned any ISBN reference, are being issued with ISBN-13 codes beginning with "979", thus easily distinguishing them.

If you're really keen to learn more about ISBN references and the Interational ISBN agency, then the links below are sure to be of benefit. These will direct you to various chapters of the online ISBN User's Manual.